A depolarizer, also called a polarization scrambler, is an optical device that reduces and preferably removes polarization of a beam of light that passes through the depolarizer. A depolarizer may be realized for example by means of a wedge shaped plate, that is, a plate that has a thickness in the beam direction that varies as a function of position in a plane transverse to the beam direction. The plate is made of a material that changes polarization by causing a relative phase shift between polarization components, by an amount that is proportional to the distance that the light travels through the plate, typically as a periodic function of the thickness. Because of the position dependent thickness the change of polarization varies as a function of position in said plane transverse to the beam, with the result that the spatial average of the polarization of the beam is reduced. The best depolarization result is achieved if the changes of the polarization at the thinnest and thickest part of the plate differ by many periods of the thickness dependence.
US2009/0296066 discloses depolarizers for use in a microlithographic projection exposure apparatus. A Hanle depolarizer is described, which contains a wedge plate of birefringent material arranged with the crystal optic axis of the birefringent material at an angle of 45 degrees with the polarization direction. As is known per se, a birefringent material has a different speed of light for light with polarization directed along a main crystal axis than for light with polarization directed transverse to that axis, thus causing the phase shift. A Hanle depolarizer is most effective when the incoming light has a substantial polarization component at an angle of 45 degrees with the crystal optic axis. It is ineffective for the polarization state directed at 0 or 90 degrees angle. To bring about a relative phase change for these components, a first and second wedge may be used, with the birefringent axes of the first and second wedge oriented at forty five degrees to each other.
US2009/0296066 also describes a depolarizer with wedge plates of optically active material wherein the optic axis is directed along the beam direction. Herein the speed of light for different helical polarizations around the optical axis differs. Furthermore, US2009/0296066 discloses an embodiment wherein these wedge shape plate is replaced by a plate with a sawtooth surface, which effectively provides a plurality of local wedge shapes, with back to back increases and decreases of the thickness. This structure makes it possible to provide a high thickness gradient, that is, a high polarization rotation gradient, without requiring a plate that is very thick at its thickest point.
The use of a wedge shape, i.e. a plate wherein the opposing surfaces are not parallel has the effect of deflecting the optical beam, which is undesirable. Therefore, the Hanle depolarizer usually contains a first and second wedge plate in series, the first and second wedge shaped plate conventionally being of the same material, with crystallographic axes rotated relative to each other. The resulting depolarizer is also called a Babinet depolarizer. In such a depolarizer, the input plane of the first wedge plate is parallel to the output plane of the second wedge plate and perpendicular to the beam direction. Internally in the depolarizer the output plane of the first wedge plate is parallel to the input plane of the second wedge plate, both at a non-perpendicular angle to the beam direction. Two such pairs of wedges may be used if wedges with birefringent axes oriented at forty five degrees to each other are used. This results in a Dual Babinet Compensator Pseudo-depolarizer (DBCP). Likewise, first and second wedge plates of optically active material may be used in series, with opposite polarization dependent differences. US2009/0296066 also describes a pairs of plate with complementary saw tooth surfaces, the teeth of the first plate being inserted between the teeth of the second plate.
In each of these embodiments, the depolarizer has parallel input and output surfaces, so that a single beam direction can be orthogonally incident on both surfaces. However, there is still an oblique interface within the pair of wedges with differently oriented material that has slightly different speeds of light for corresponding polarizations on different sides of the oblique interface, as is needed for the depolarizing effect. This means that a slight deflection of the beam is inevitable. What is worse is that this deflection differs for different polarization components. When used in a telescope the depolarizer therefore has the effect of splitting a spot image, or at least blurring the spot. For example, with four wedges, such as in a DBCP depolarizer, a spot may be split into 24=16 parts. Yet worse, the intensity of the different spot parts depends on the intensity of the different polarization components at the input. Thus, the centre of mass of the spot (the weighted average of the positions in the spot, weighted by intensity) may shift when the intensity distribution of the polarization components at the input varies. Although this shift need not be detrimental to the resolution of the telescope, it can affect the reliability of measurements performed at specific points in the image plane.
U.S. Pat. No. 4,198,123 discloses a Babinet depolarizer comprising two wedge shaped elements joined at their oblique surfaces so that the front and back surface are parallel planes.